This invention relates to surface wave devices and, more particularly, to a surface wave variable delay device fabricated on ferroelectric materials and a method and apparatus for making same. The subject matter of this application is related to subject matter disclosed in the copending application Ser. No. 430,030 entitled "Surface Wave Transducer and Method and Apparatus For Making Same" filed of even date herewith and assigned to the same assignee as the present application. and now U.S. Pat. No. 3,924,145, issued Dec. 2, 1976.
Ferroelectric materials generally exhibit characteristics of both ferroelectricity and piezoelectricity. However, ceramic ferroelectric materials, such as barium titanate and lead titanate zirconate, exhibit little or no piezoelectric effect unless they are first "poled." It is believed that the absence of piezoelectricity in these materials is due to the nature of their "ferroelectric domain" structure. Specifically, each domain can be thought of as having its own intrinsic polarization direction with the directions being essentially random. When an electric field is applied to the material a particular domain may be elongated while its neighboring domain, whose spontaneous polarization direction is of opposite sense, will be contracted. The result is that the overall material exhibits little net piezoelectricity.
It is well known that polycrystalline ferroelectric ceramics can be rendered piezoelectric by reorienting the material's domains in the same direction; i.e., by "poling." The common technique of poling is to apply a strong electric field to the material for a relatively long period, usually at an elevated temperature.
After a ferroelectric material has been poled, a surface wave device can be fabricated by forming transducers thereon which are capable of converting electric signals into surface elastic waves or vice versa. A particularly efficient transducer comprises a pair of interdigitated comb-like metal electrodes which are generally formed on the ceramic substrate surface using photolithographic techniques. When a voltage is applied to the electrodes the resultant electric field between each pair of adjacent fingers gives rise to an elastic wave which propagates along the material's surface. The fingers of the comb electrodes have spacings selected such that the waves generated by each finger pair add coherently. Similarly, a travelling elastic wave gives rise to a varying electric field which can be sensed with interdigitated electrodes acting as a receiver.
It has recently been demonstrated that interdigitated electrodes can be formed on the surface of an unpolarized ferroelectric material and the region below the electrodes poled by applying a strong D.C. field across the electrodes. After this "selective" poling is achieved, the electrodes are used to excite surface acoustic waves in the material. In such case, the attained polarization is of the same pattern as the electric field used for exciting surface waves.
The above-described techniques, while useful and operative, include various inherent disadvantages. For example, the photolithographic techniques employed to form the small delicate electrode fingers involve undue time and expense. Also, once the interdigitated electrodes are formed, tiny geometries limit the voltage level which can be applied during device operation. More specifically, since the electric fields applied across the electrodes extend into the environment above the substrate, such fields are limited by the dielectric strength of air if breakdown is to be avoided.
It is an object of this invention to provide improvements in the art which are responsive to the stated problems among others.